Coating compositions and method of forming coating film

ABSTRACT

The present invention provides a thermosetting and photocurable coating composition comprising: (A) at least one compound selected from the group consisting of radical-polymerizable unsaturated monomers, resins containing radical-polymerizable unsaturated groups, and resins containing radical-polymerizable unsaturated groups and thermosetting functional groups; (B) a hydroxyl-containing polyester resin produced by esterifying a polybasic acid (a) and a polyhydric alcohol (b), wherein an alicyclic polybasic acid (a1) and/or an alicyclic polyhydric alcohol (b1) are included in a ratio of 20% or more based on the total weight of polybasic acid (a) and polyhydric alcohol (b); (C) a crosslinking agent; and (D) a photopolymerization initiator; and a method of forming a coating film using the coating composition.

TECHNICAL FIELD

The present invention relates to a coating composition and a method offorming a coating film.

BACKGROUND ART

For the formation of a coating film on the bodies of vehicles such asautomobiles, motorcycles, container vehicles and the like, a method iswidely used which comprises applying a thermosetting coating compositionthat contains a resin having thermosetting functional groups, such as ahydroxyl-containing acrylic resin, and a crosslinking agent, such as amelamine resin, to the vehicle body to be coated, and thereafter curingthe applied coating composition by heating. This method can form acoating film having excellent film properties in terms of adhesion,hardness, etc.

In recent years, the above-mentioned coating film formation method hasbeen required to reduce energy consumption and increase productivity.For example, the heat-curing step on an automobile body coating line isusually conducted at about 140° C. for about 40 minutes; when theconveyor runs at a speed of 3 m/min, the step requires a space for thedrying furnace line about 120 m in length. Therefore, in order to savespace and energy, shortening of the heating time is demanded in theheat-curing step. Moreover, the reduction of the heating time ispreferable for decreasing CO₂, soot, etc., which cause environmentalproblems.

In addition, the coating film formed on an automobile body is nowincreasingly required to have improved resistance to marring by carwashes, scratching around keyholes, etc. in view of better appearance.It has thus become important to develop a coating composition that canform a coating film on an automobile body or the like, having excellentresistance to marring by car washes, scratching, etc.

To shorten the heating time in the formation of a coating film, JapaneseUnexamined Patent Publications No. 1988-113085 and No. 1989-11169disclose a method of coating a substrate with an ultraviolet-curable andthermosetting coating composition containing ultraviolet-curablepolyfunctional (meth)acrylates, polyhydric alcohol mono(meth)acrylatepolymers, and polyisocyanate compounds or melamine resins, etc., andthen carrying out ultraviolet irradiation, followed by about 5 to about30 minutes of heat-curing to form a coating film. However, although themethod using this coating composition can shorten the heating time, themethod cannot form a cured coating film having excellent mar resistance.

U.S. Pat. No. 5,425,970 discloses a coating film formation method thatcomprises forming a colored base coat on a substrate to be coated,coating the base coat with a thermosetting clear coating composition,heat-curing the resulting coating film, further coating the film with aradiation-curable clear coating composition containingradical-polymerizable binders such as (meth)acrylic-functional(meth)acrylic copolymers, epoxy resin (meth)acrylates, polyester(meth)acrylates, etc. or cationically polymerizable binders such aspolyfunctional epoxy oligomers, etc. and irradiating the film to cureit. However, the method using this radiation-curable clear coatingcomposition, although capable of reducing the total heating time, cannotform a cured coating film having excellent mar resistance.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a coating compositionthat enables shortening of the heating time for forming a cured coatingfilm, thereby contributing to saving space and energy as well asreducing CO₂ and other agents of environmental damage; and forming acoating film having excellent properties in mar resistance, hardness,adhesion, etc.

Another object of the invention is to provide a method of forming acoating film using the above coating composition.

Further objects and features of the invention will be apparent from thefollowing description.

In order to achieve the above objects, the present inventors conductedextensive research. As a result, the inventors found that when athermosetting and photocurable coating composition containing a specificradical-polymerizable compound, a specific polyester resin, acrosslinking agent and a photopolymerization initiator is used, theheating time for curing a coating film can be reduced by the addition ofa photocuring step. The inventors also found that the resulting curedcoating film has high elasticity as well as excellent hardness, thusproviding excellent mar resistance, and that the film has other superiorproperties in terms of adhesion, etc. Based on these novel findings, theinventors carried out further research and accomplished the presentinvention.

The present invention provides a coating composition and a method offorming a coating film using the coating composition as follows.

Item 1: A thermosetting and photocurable coating composition comprising:

-   -   (A) at least one compound selected from the group consisting of        radical-polymerizable unsaturated monomers, resins containing        radical-polymerizable unsaturated groups, and resins containing        radical-polymerizable unsaturated groups and thermosetting        functional groups;    -   (B) a hydroxyl-containing polyester resin produced by        esterifying a polybasic acid (a) with a polyhydric alcohol (b),        wherein an alicyclic polybasic acid (a1) and/or an alicyclic        polyhydric alcohol (b1) are included in a ratio of 20% or more        based on the total weight of polybasic acid (a) and polyhydric        alcohol (b);    -   (C) a crosslinking agent; and    -   (D) a photopolymerization initiator.

Item 2: A coating composition according to item 1, wherein the polybasicacid (a) includes 50% or more of alicyclic polybasic acid (a1) based onthe weight of polybasic acid (a).

Item 3: A coating composition according to item 1, wherein thepolyhydric alcohol (b) includes 50% or more of alicyclic polyhydricalcohol (b1) based on the weight of polyhydric alcohol (b).

Item 4: A coating composition according to item 1, wherein the polyesterresin (B) has a hydroxyl value of 20 to 800 mg KOH/g.

Item 5: A coating composition according to item 1, wherein thecrosslinking agent (C) is a polyisocyanate compound.

Item 6: A coating composition according to item 1, wherein thecrosslinking agent (C) is a combination of a polyisocyanate compound anda melamine resin.

Item 7: A coating composition according to item 1, comprising, per 100parts by weight of polyester resin (B), about 5 to about 200 parts byweight of compound (A), about 5 to about 200 parts by weight ofcrosslinking agent (C) and about 0.1 to about 20 parts by weight ofphotopolymerization initiator (D).

Item 8: A coating composition according to item 1, further comprising alight stabilizer (E).

Item 9: A coating composition according to item 1, further comprising anultraviolet absorber (F).

Item 10: A coating composition according to item 1, wherein the coatingcomposition is an organic solvent-based composition and has a solidscontent of about 20% to about 90% by weight.

Item 11: A method of forming a coating film comprising forming one ormore colored base coats and one or more clear coats on a substrate to becoated to form a multilayer coating film, the top clear coat beingformed from the coating composition according to item 1.

Item 12: A method of forming a coating film according to item 11,wherein a colored base coat and a top clear coat are formed on thesubstrate to form a multilayer coating film according to a two-coatsystem.

Item 13: A method of forming a coating film according to item 11,wherein a colored base coat, a clear coat and a top clear coat areformed on the substrate to form a multilayer coating film according to athree-coat system.

Item 14: A method of forming a coating film according to item 11,wherein a first colored base coat, a second colored base coat and a topclear coat are formed on the substrate to form a multilayer coating filmaccording to a three-coat system.

Item 15: A method of forming a coating film according to item 11,wherein the top clear coat is cured either by irradiating with lightafter heating or by heating after irradiating with light.

Item 16: A method of forming a coating film according to item 15,wherein the top clear coat is heated at about 100° C. to about 180° C.for about 5 to about 30 minutes.

Item 17: A method of forming a coating film according to item 15,wherein the top clear coat is irradiated with ultraviolet light having awavelength of about 200 to about 450 nm at an intensity of about 100 toabout 5,000 mJ/cm².

Item 18: A method of forming a coating film according to item 11,wherein the substrate to be coated is a vehicle body.

Item 19: A method of forming a coating film according to item 18,wherein the substrate to be coated is an automobile body.

Item 20: A vehicle body on which a coating film is formed by the methodof forming a coating film according to item 11.

Item 21: An automobile body on which a coating film is formed by themethod of forming a coating film according to item 11.

The coating composition and the method of forming a coating film on asubstrate according to the present invention are described below indetail.

Thermosetting and Photocurable Coating Composition

The thermosetting and photocurable coating composition of the presentinvention comprises the above-mentioned compound (A),hydroxyl-containing polyester resin (B), crosslinking agent (C) andphotopolymerization initiator (D).

Compound (A)

The compound (A) is at least one compound selected from the groupconsisting of radical-polymerizable unsaturated monomers, and resinscontaining radical-polymerizable unsaturated groups and thermosettingfunctional groups.

A radical-polymerizable unsaturated monomer has one or moreradical-polymerizable unsaturated groups per molecule. Such monomers areselected from the group consisting of monofunctional polymerizablemonomers having one radical-polymerizable unsaturated group permolecule, bifunctional polymerizable monomers having tworadical-polymerizable unsaturated groups per molecule, andpolyfunctional polymerizable monomers having three or moreradical-polymerizable unsaturated groups per molecule. These monomersmay be used singly or in combination of two or more. Examples thereofare given below.

Examples of monofunctional polymerizable monomers include styrene,methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate,2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, cyclohexyl(meth)acrylate, cyclohexenyl (meth)acrylate, 2-hydroxyl (meth)acrylate,hydroxypropyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate,ε-caprolactone-modified tetrahydrofurfuryl (meth)acrylate, phenoxyethyl(meth)acrylate, phenoxy polyethylene glycol (meth)acrylate,dicyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate,isobornyl (meth)acrylate, benzyl (meth)acrylate, ε-caprolactone-modifiedhydroxyethyl (meth)acrylate, polyethylene glycol mono(meth)acrylate,polypropylene glycol mono(meth)acrylate, 2-hydroxy-3-phenoxypropyl(meth)acrylate, 2-hydroxy-3-butoxypropyl (meth)acrylate, phthalic acidmonohydroxyethyl (meth)acrylate, para-cumylphenol ethyleneoxide-modified (meth)acrylate, N-methylol (meth)acrylamide, N-methylol(meth)acrylamide butyl ether, acryloyl morpholine, dimethylaminoethyl(meth)acrylate, N-vinyl-2-pyrrolidone, etc.

Examples of bifunctional polymerizable monomers include ethylene glycoldi(meth)acrylate, diethylene glycol di(meth)acrylate, polyethyleneglycol di(meth)acrylate, propylene glycol di(meth)acrylate, dipropyleneglycol di(meth)acrylate, polypropylene glycol di(meth)acrylate,neopentyl glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate,1,6-hexanediol di(meth)acrylate, bisphenol-A ethylene oxide-modifieddi(meth)acrylate, bisphenol-A propylene oxide-modified di(meth)acrylate,2-hydroxy-l-acryloxy-3-methacryloxypropane, tricyclodecane dimethanoldi(meth)acrylate, di(meth)acryloyloxyethyl acid phosphate, etc. Usablebifunctional monomers include those monomers that are commerciallyavailable under the trade names of “KAYARAD HX-220,” “KAYARAD HX-620,”“KAYARAD R-604,” “MANDA,” and so on (products of Nippon Kayaku Co.,Ltd.).

Examples of polyfunctional polymerizable monomers having three or moreradical-polymerizable unsaturated groups per molecule includetrimethylolpropane tri(meth)acrylate, trimethylolpropane ethyleneoxide-modified tri(meth)acrylate, trimethylolpropane propyleneoxide-modified tri(meth)acrylate, glycerin tri(meth)acrylate, glycerinethylene oxide-modified tri(meth)acrylate, glycerin propyleneoxide-modified tri(meth)acrylate, pentaerythritol tri(meth)acrylate,pentaerythritol tetra(meth)acrylate, isocyanuric acid ethyleneoxide-modified triacrylate, dipentaerythritol hexa(meth)acrylate, etc.

To enhance the curability of the coating composition, mar resistance ofthe cured coating film, etc., preferable radical-polymerizableunsaturated monomers are those having two or more radical-polymerizableunsaturated groups per molecule.

Examples of resins containing radical-polymerizable unsaturated groupsinclude unsaturated acrylic resins, unsaturated urethane resins,unsaturated epoxy resins, polyester (meth)acrylates, unsaturatedsilicone resins, etc. These resins may be used singly or in combinationof two or more.

A resin containing radical-polymerizable unsaturated groups andthermosetting functional groups is a resin that has one or moreradical-polymerizable unsaturated groups and one or more thermosettingfunctional groups per molecule. In view of better curability of thecoating composition, the resin preferably contains two or moreradical-polymerizable unsaturated groups and two or more thermosettingfunctional groups per molecule. Examples of thermosetting functionalgroups are hydroxyl group, acid groups, epoxy group, isocyanate group,and like functional groups. Examples of the acid groups include carboxylgroup, phosphate group, etc.

Examples of resins containing radical-polymerizable unsaturated groupsand thermosetting functional groups include acrylic resins containingradical-polymerizable unsaturated groups and hydroxyl groups, acrylicresins containing radical-polymerizable unsaturated groups and carboxylgroups, acrylic resins containing radical-polymerizable unsaturatedgroups and epoxy groups, acrylic resins containing radical-polymerizableunsaturated groups and isocyanate groups, polyester resins containingradical-polymerizable unsaturated groups and hydroxyl groups, polyesterresins containing radical-polymerizable unsaturated groups and carboxylgroups, cresol novolac epoxy resins containing radical-polymerizableunsaturated groups and epoxy groups, etc. These resins may be usedsingly or in combination of two or more.

Hydroxyl-containing Polyester Resin (B)

The hydroxyl-containing polyester resin (B) is produced by esterifyingpolybasic acid(s) (a) and polyhydric alcohol(s) (b) by conventionalprocedures. Direct esterification methods and transesterificationmethods are usable for the esterification reaction.

In the preparation of polyester resin (B), the polybasic acid(s) (a) andthe polyhydric alcohol(s) (b) include alicyclic polybasic acid(s) (a1)and/or alicyclic polyhydric alcohol(s) (b1) in a proportion of 20% ormore, and preferably 50% to 100%, based on the total weight of polybasicacid(s) (a) and polyhydric alcohol(s) (b). This proportion allows theformation of a coating film having excellent properties in terms of marresistance, hardness, adhesion, etc. and makes it easy to shorten theheating time for curing the coating film. When the proportion is lessthan 20% by weight, the resulting coating film becomes unsatisfactory inits hardness and adhesion.

The proportion of alicyclic polybasic acid (a1) in polybasic acid (a) ispreferably 50% or more by weight, and more preferably 50% to 100% byweight. The proportion of alicyclic polyhydric alcohol (b1) inpolyhydric alcohol (b) is preferably 50% or more by weight, and morepreferably 50% to 100% by weight. When the proportions of alicyclicpolybasic acid (a1) in polybasic acid (a) and alicyclic polyhydricalcohol (b1) in polyhydric alcohol (b) are each 50% or more by weight, acoating film can be formed having excellent properties in marresistance, hardness, adhesion, etc., and the heating time for curingthe coating film can be more easily shortened.

Herein, the proportions of alicyclic polybasic acid (a1) and alicyclicpolyhydric alcohol (b1) are all expressed on a solids basis.

Examples of polybasic acids (a) include dibasic acids such as phthalicanhydride, isophthalic acid, terephthalic acid,cyclohexane-1,3-dicarboxylic acid, cyclohexane-1,4-dicarboxylic acid,hexahydrophthalic acid, hexahydroisophthalic acid, hexahydroterephthalicacid, hexahydrotrimellitic acid, tetrahydrophthalic acid, methylhexahydrophthalic acid, tetrahydrophthalic anhydride, hexahydrophthalicanhydride, succinic acid, fumaric acid, adipic acid, sebacic acid,maleic anhydride, etc.; lower alkyl esters of dibasic acids; tribasic orhigher polybasic acids such as trimellitic anhydride, methylcyclohexenetricarboxylic acid, pyromellitic anhydride, etc.; and so on.

Of the above polybasic acids (a), alicyclic polybasic acid(s) (a1) areused in the predetermined amount. A preferable alicyclic polybasic acid(a1) is one that has one or two 4- to 6-membered rings or like alicyclicstructures and two or more carboxyl groups per molecule. Examples ofsuch preferable alicyclic polybasic acids (a1) includecyclohexane-1,3-dicarboxylic acid, cyclohexane-1,4-dicarboxylic acid,hexahydrophthalic acid, hexahydroisophthalic acid, hexahydroterephthalicacid, hexahydrotrimellitic acid, tetrahydrophthalic acid, methylhexahydrophthalic acid; anhydrides of these acids; etc.

One or more polybasic acids selected from the above dibasic acids andlower alkyl esters thereof are mainly used as the polybasic acid (a),with tribasic or higher polybasic acids being optionally used.

If necessary, the above polybasic acids (a) may be used in combinationwith monobasic acids such as benzoic acid, crotonic acid,p-t-butylbenzoic acid, etc. for molecular weight adjustment, etc. Oilfatty acids such as coconut oil fatty acid, dehydrated castor oil fattyacid, etc. may also be used.

Usable polyhydric alcohols (b) include dihydric alcohols, which have twohydroxyl groups per molecule, and polyhydric alcohols having three ormore hydroxyl groups per molecule.

Examples of dihydric alcohols include glycols such as ethylene glycol,propylene glycol, diethylene glycol, trimethylene glycol, tetraethyleneglycol, triethylene glycol, dipropylene glycol, 1,4-butanediol,1,3-butanediol, 2,3-butanediol, 1,2-butanediol, 3-methyl-1,2-butanediol, 1,2-pentanediol, 1,5-pentanediol, 1,4-pentanediol,2,4-pentanediol, 2,3-dimethyltrimethylene glycol, tetramethylene glycol,3-methyl-4,5-pentanediol, 2,2,4-trimethyl-1,3-pentanediol,1,6-hexanediol, 1,5-hexanediol, 1,4-hexanediol, 2,5-hexanediol,neopentyl glycol, hydroxypivalic acid neopentyl glycol ester, etc.;polylactone diols obtained by the addition of lactones such asε-caprolactone or the like to these glycols; polyester diols such asbis(hydroxyethyl) terephthalate, etc.; alicyclic dihydric alcohols suchas cyclohexane-1,4-dimethylol, hydrogenated bisphenol-A, spiroglycol,dihydroxymethyltricyclodecane, etc.; and so on.

Examples of polyhydric alcohols having three or more hydroxyl groups permolecule include glycerin, trimethylolpropane, trimethylolethane,diglycerin, triglycerin, 1,2,6-hexanetriol, pentaerythritol,dipentaerythritol, sorbitol, mannitol, etc.

Of the above polyhydric alcohols (b), alicyclic polyhydric alcohol(s)(b1) are used in the predetermined amount. A preferable alicyclicpolyhydric alcohol (b1) is one that has one or two 4- to 6-memberedrings or like alicyclic structures and two or more hydroxyl groups permolecule. Examples of such preferable alicyclic polyhydric alcohols (b1)include cyclohexane-1,4-dimethylol, hydrogenated bisphenol-A,spiroglycol, dihydroxymethyltricyclodecane, etc.

The hydroxyl-containing polyester resin (B) prepared from the abovestarting materials has a weight average molecular weight of usuallyabout 500 to about 500,000, preferably about 1,000 to about 100,000,more preferably about 2,000 to 50,000; a hydroxyl value of about 20 toabout 800 mg KOH/g, preferably about 80 to about 200 mg KOH/g; and anacid value of about 4 to about 200 mg KOH/g, preferably about 4 to about100 mg KOH/g.

Crosslinking Agent (C)

The crosslinking agent (C) is a compound that undergoes a crosslinkingreaction with the thermosetting functional groups of the compound (A)and the hydroxyl groups of the hydroxyl-containing polyester resin (B).Examples of such crosslinking agents (C) include polyisocyanatecompounds, melamine resins, guanamine resins, urea resins, etc.

To obtain a coating film having excellent properties in mar resistance,hardness, adhesion, etc., it is preferable to use a polyisocyanatecompound alone or in combination with a melamine resin. When apolyisocyanate compound and a melamine resin are used in combination,the polyisocyanate compound/melamine resin weight ratio is within therange of about 10/90 to about 90/10 on a solids basis.

The polyisocyanate is a compound having two or more free isocyanategroups per molecule. Examples thereof include organic polyisocyanates,including aliphatic diisocyanates such as hexamethylene diisocyanate,trimethyl hexamethylene diisocyanate, dimer acid diisocyanate, lysinediisocyanate, and the like; alicyclic diisocyanates such as hydrogenatedxylylene diisocyanate, cyclohexylene diisocyanate,methylenebis(cyclohexyl isocyanate), isophorone diisocyanate, and thelike; aromatic diisocyanates such as tolylene diisocyanate, phenylenediisocyanate, 4,4′-diphenylmethane diisocyanate, xylylene diisocyanate,tetramethylxylylene diisocyanate, naphthalene diisocyanate, and thelike; trivalent or higher organic polyisocyanate compounds such as2-isocyanatoethyl-2,6-diisocyanatocaproate,3-isocyanatomethyl-1,6-hexamethylene diisocyanate,4-isocyanatomethyl-1,8-octamethylene diisocyanate (commonly referred toas triamino-nonane triisocyanate), and the like; etc.

Other usable polyisocyanate compounds are dimers and trimers of theabove polyisocyanate compounds; prepolymers formed by the urethanationreaction of polyisocyanate compounds with polyhydric alcohols,low-molecular-weight polyester resins, water, etc. in the presence of anexcess of isocyanates; and so on.

Polyisocyanate compounds may be blocked by a blocking agent. Examples ofblocking agents include phenols, oximes, lactams, alcohols, mercaptanes,active methylene compounds such as diethyl malonate, etc. A blockedpolyisocyanate compound is preferably used in combination with acatalyst for the dissociation of the blocking agent.

Unblocked polyisocyanate compounds and blocked polyisocyanate compoundsmay be used in combination.

A preferable melamine resin is, for example, obtained by etherifyingpart of or all of the methylol groups in a methylolated melamine with amonoalcohol having 1 to 8 carbon atoms. Preferably, the melamine resinhas about 1 to about 5 triazine rings and a number average molecularweight of about 300 to about 2,000.

In the etherified melamine resin, the methylol groups in themethylolated melamine may be fully etherified, or may be partiallyetherified with some methylol groups or imino groups remaining. Examplesof etherified melamine resins include alkyl-etherified melamine resinssuch as methyl-etherified melamine, ethyl-etherified melamine,butyl-etherified melamine, etc. Such etherified melamine resins may beused singly or in combination of two or more.

Photopolymerization Initiator (D)

The coating composition of the present invention contains aphotopolymerization initiator (D) as an essential component. Examples ofthe photopolymerization initiator (D) include benzoin, benzoin methylether, benzoin ethyl ether, benzoin isobutyl ether,diethoxyacetophenone, 2-hydroxy-2-methyl-l-phenylpropane-1-one, benzyldimethyl ketal, 1-hydroxycyclohexyl-phenylketone,2-methyl-2-morpholino(4-thiomethylphenyl)propane-1-one,2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone,2,4,6-trimethylbenzoylphenylphosphine oxide,bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide,2,4,6-trimethylbenzoylphenylethoxyphosphine oxide, benzophenone, methylo-benzoylbenzoate, hydroxybenzophenone, 2-isopropylthioxanthone,2,4-dimethylthioxanthone, 2,4-diethylthioxanthone,2,4-dichlorothioxanthone, 2,4,6-tris(trichloromethyl)-s-triazine,2-methyl-4,6-bis(trichloro)-s-triazine,2-(4-methoxyphenyl)-4,6-bis(trichloromethyl)-s-triazine, etc.

Such photopolymerization initiators (D) may be used singly or incombination of two or more.

The photopolymerization initiators (D) may be used in combination withphotosensitizers to accelerate the photopolymerization reaction.Examples of such photosensitizers include tertiary amines such astriethylamine, triethanolamine, methyldiethanolamine, methyl4-dimethylaminobenzoate, ethyl 4-dimethylaminobenzoate, isoamyl4-dimethylaminobenzoate, (2-dimethylamino)ethyl benzoate, Michler'sketone, 4,4′-diethylaminobenzophenone, etc.; alkylphosphines such astriphenylphosphine, etc.; thioethers such as β-thiodiglycol, etc.; andso on.

The thermosetting and photocurable coating composition of the presentinvention comprises a compound (A), a hydroxyl-containing polyesterresin (B), a crosslinking agent (C) and a photopolymerization initiator(D) as essential components, preferably in the following proportions.

The coating composition of the invention preferably comprises, per 100parts by weight of polyester resin (B), about 5 to about 200 parts byweight of compound (A), about 5 to about 200 parts by weight ofcrosslinking agent (C) and about 0.1 to about 20 parts by weight ofphotopolymerization initiator (D).

When the coating composition contains compound (A), hydroxyl-containingpolyester resin (B), crosslinking agent (C) and photopolymerizationinitiator (D) in the above specified proportion, the composition usuallyhas satisfactory thermosetting and photocuring properties.

More preferably, the coating composition of the invention comprises per100 parts by weight of polyester resin (B), about 5 to about 100 partsby weight of compound (A), about 5 to about 100 parts by weight ofcrosslinking agent (C) and about 0.1 to about 10 parts by weight ofphotopolymerization initiator (D).

Light Stabilizer (E)

The coating composition of the present invention may optionally containa light stabilizer to enhance the weatherability of the coating film.The light stabilizer is used as a radical chain inhibitor to capture theactive radical species generated in the course of deterioration of acoating film. Examples of such light stabilizers include hindered aminelight stabilizers. The light stabilizer can optionally be used incombination with an ultraviolet absorbers (F), which is described later.

Light stabilizers having excellent light stabilization properties are,for example, hindered piperidines. Examples of hindered piperidines aremonomeric hindered piperidines such asbis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate,bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate,bis(N-methyl-2,2,6,6-tetramethyl-4-piperidinyl) sebacate,4-benzoyloxy-2,2′,6,6′-tetramethylpiperidine,bis(1,2,2,6,6-pentamethyl-4-piperidyl){[3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl]methyl}butylmalonate, etc.; oligomeric hindered piperidines such aspoly{[6-(1,1,3,3-tetramethylbutyl)imino-1,3,5-triazine-2,4-diyl][(2,2,6,6-tetramethyl-4-piperidyl)imino]hexamethylene[(2,2,6,6-tetramethyl-4-piperidyl)iminol]},etc.; and polyester hindered piperidines such as polyesters of4-hydroxy-2,2,6,6-tetramethyl-l-piperidineethanol and succinic acid,etc. Light stabilizers are not limited to these hindered piperidines.

When used in the thermosetting and photocurable coating composition ofthe invention, the light stabilizer (E) is usually used in a ratio ofabout 0.1% to about 10% by weight relative to the solids content of thecoating composition.

Ultraviolet Absorber (F)

The coating composition of the present invention may optionally comprisean ultraviolet absorber to increase the weatherability of a coatingfilm. The ultraviolet absorber absorbs incident light and converts lightenergy into a harmless form like heat energy to inhibit the start ofdeterioration of the coating film. The ultraviolet absorber can be usedin combination with the above light stabilizer (E).

Known ultraviolet absorbers can be used, including benzotriazoleabsorbers, triazine absorbers, salicylic acid derivative absorbers,benzophenone absorbers, etc.

Examples of benzotriazole absorbers include2-(2′-hydroxy-5′-methylphenyl) benzotriazole,2-(2′-hydroxy-5′-t-butylphenyl)benzotriazole,2-(2′-hydroxy-3′,5′-di-t-butylphenyl)benzotriazole,2-(2′-hydroxy-3′-t-butyl-5′-methylphenyl)-5-chlorobenzotriazole,2-(2′-hydroxy-3′,5′-di-t-butylphenyl)-5-chlorobenzotriazole,2-(2′-hydroxy-3′, 5′-di-t-amylphenyl)benzotriazole,2-(2′-hydroxy-4′-octoxyphenyl)benzotriazole,2-{2′-hydroxy-3′-(3″,4″,5″,6″-tetrahydrophthalimidemethyl)-5′-methylphenyl}benzotriazole,etc.

Examples of triazine absorbers include2,4-bis(2,4-dimethylphenyl)-6-(2-hydroxy-4-isooctyloxyphenyl)-1,3,5-triazine,2-[4((2-hydroxy-3-dodecyloxypropyl)-oxy)-2-hydroxyphenyl]-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine,2-[4-((2-hydroxy-3-tridecyloxypropyl)-oxy)-2-hydroxyphenyl]-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine2-(2,4-dihydroxyphenyl)-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine, etc.

Examples of salicylic acid derivative absorbers include phenylsalicylate, p-octylphenylsalicylate, 4-tert-butylphenylsalicylate, etc.

Examples of benzophenone absorbers include 4-dihydroxybenzophenone,2-hydroxy-4-methoxybenzophenone, 2,2′-dihydroxy-4-methoxybenzophenone,2-hydroxy-4-methoxy-2′-carboxybenzophenone,2-hydroxy-4-methoxy-5-sulfobenzophenone trihydrate,2,2′-dihydroxy-4,4′-dimethoxybenzophenone,2-hydroxy-4-octoxybenzophenone, 2-hydroxy-4-octadecyloxybenzophenone,sodium 2,2′-dihydroxy-4, 4′-dimethoxy-5-sulfobenzophenone,2,2′,4,4′-tetrahydroxybenzophenone, 4-dodecyloxy-2-hydroxybenzophenone,5-chloro-2-hydroxybenzophenone, resorcinol monobenzoate, 2,4-dibenzoylresorcinol, 4,6-dibenzoyl resorcinol, hydroxydodecylbenzophenone,2,2′-dihydroxy-4(3-methacryloxy-2-hydroxypropoxy)benzophenone, etc.

When used in the thermosetting and photocurable coating composition ofthe invention, the ultraviolet absorber (F) is usually incorporated in aproportion of about 0.1% to about 10% by weight relative to the solidscontent of the coating composition.

To improve workability and the finish quality of a coating film, thethermosetting and photocurable coating composition of the invention ispreferably in the form of an organic solvent-based composition.

Examples of usable organic solvents include aromatic solvents such astoluene, xylene, etc.; ester solvents such as ethyl acetate, propylacetate, butyl acetate, methoxybutyl acetate, amyl acetate, methylcellosolve acetate, cellosolve acetate, diethylene glycol monomethylether acetate, carbitol acetate, etc.; ether solvents such as dioxane,ethylene glycol diethyl ether, ethylene glycol dibutyl ether, etc.;ketone solvents such as acetone, methyl ethyl ketone, methyl isobutylketone, etc.; and so on. These organic solvents may be used singly or incombination of two or more.

To improve the finish quality of a coating film, the organic solvent isused in the coating composition of the invention preferably in such anamount that the coating composition has a solids content of about 20% toabout 90% by weight, and more preferably about 30% to about 70% byweight.

The coating composition of the invention may be a clear coatingcomposition or may be a colored coating composition containing acoloring pigment and/or an effect pigment, etc. The coating compositionmay optionally contain other pigments such as extender pigments.

Examples of coloring pigments include inorganic pigments such astitanium dioxide, iron oxides, etc.; organic pigments such asphthalocyanine blue, quinacridone red, perylene red, phthalocyaninegreen, etc.; and so on. Examples of effect pigments include aluminumflakes, mica flakes, etc. Examples of extender pigments include bariumsulfate, calcium carbonate, talc, clay, etc.

If necessary, known additives such as surface modifiers, anti-saggingagents, anti-settling agents, plasticizers, etc. may be added to thecoating composition of the invention.

Method of Forming a Coating Film

The coating composition of the present invention can be suitably used invarious methods of forming a coating film on a substrate, as describedhereinafter.

Substrate to be Coated

Although the substrate to be coated in the method of the presentinvention is not limited, the bodies of vehicles such as automobiles,motorcycles, container vehicles and the like are preferable. Otherexamples of substrates to be coated are materials for vehicle bodycomponents, including metal substrates such as aluminum sheets andplates, aluminum alloy sheets and plates, steel sheets and plates suchas cold-rolled steel sheets and plates, galvanized steel sheets andplates, zinc alloy-plated steel sheets and plates, stainless steelsheets and plates, tin-plated steel sheets and plates, etc.; variousplastic substrates; and so on.

The metal surface of a vehicle body or metal substrate to be coated maybe pretreated by phosphating, chromating, mixed oxide treatment, etc.Further, the vehicle body, metal substrate, etc. to be coated may havean undercoating film and/or an intermediate coating film, which areformed from various electrodeposition coating compositions, etc.

Methods of Coating and Curing

The coating method for the composition of the present invention is notrestricted. A wet coating film can be formed by methods such as airspray coating, airless spray coating, rotary atomization coating,electrostatic coating, curtain coating, etc. In these coating methods,an electrostatic charge may be optionally applied. Of the above methods,air spray coating, electrostatic coating, etc. are preferable. Thecoating composition is usually applied to a film thickness of about 10to about 70 μm (when cured).

When carrying out air spray coating, airless spray coating or rotaryatomization coating, it is preferable to use an amount of organicsolvent to suitably adjust the viscosity of the coating compositiondepending on the coating method, which is usually within the range ofabout 15 to about 60 seconds at 20° C. (viscometer: Ford cup No. 4).

After coating a substrate with the coating composition, the resultingwet coating film is cured by irradiating with light after heating or byheating after irradiating with light, such that the film issubstantially fully cured.

Heating can be performed by known heating means. Examples thereofinclude drying furnaces such as hot air furnaces, electric furnaces,infrared induction heating furnaces, etc.

The heating temperature is usually in the range of about 50° C. to about200° C., and preferably in the range of about 70° C. to about 160° C.The heating time is usually in the range of about 5 to about 30 minutes.Under such heating conditions, a coating film can be semicured by, forexample, heating at about 140° C. for about 20 minutes in theheat-curing step on an automobile body coating line. When the conveyorspeed is 3 m/min, the step requires a space for a drying furnace linewith a length of about 60 m, which is about half the length usuallyrequired. Space and energy thus can be saved.

The irradiating light is usually ultraviolet light having a wavelengthof about 200 to about 450 nm.

A light source can be suitably selected to provide wavelengths to whicha selected photopolymerization initiator is highly sensitive. Examplesof sources of the above-mentioned ultraviolet light include highpressure mercury lamps, ultrahigh pressure mercury lamps, xenon lamps,carbon arc lamps, metal halide lamps, sunlight, etc. The coating film isirradiated with ultraviolet light at an energy intensity of preferablyabout 100 to about 5,000 mJ/cm², and more preferably about 300 to about3,000 mJ/cm². The irradiation time is usually about 3 seconds to about 3minutes. The coating film can be substantially fully cured in thisphotocuring step.

Steps of Forming a Coating Film

The coating composition of the present invention can form a coating filmhaving excellent properties in terms of mar resistance, hardness,adhesion, etc. Therefore, the coating composition of the invention ispreferably used for forming the top clear coat in the process of forminga multilayer coating film on a substrate.

The coating film formation method of the present invention comprisesforming one or more colored base coats and then one or more clear coatson a substrate to be coated to form a multilayer coating film, the topclear coat being formed from the coating composition of the invention.

For example, the coating film formation method of the present inventionmay be any of the following multilayer coating film formation methods(a) to (c), wherein the top clear coat is formed from the clear coatingcomposition of the invention.

Method (a): A two-coat method of forming a multilayer coating film,wherein a colored base coat and a top clear coat are formed on asubstrate to be coated.

Method (b): A three-coat method of forming a multilayer coating film,wherein a colored base coat, a clear coat and a top clear coat areformed in that order on a substrate to be coated.

Method (c): A three-coat method of forming a multilayer coating film,wherein a first colored base coat, a second colored base coat and a topclear coat are formed in that order on a substrate to be coated.

Moreover, the coating composition of the present invention can also beused when forming a single-layer coating film. In this case, the coatingcomposition may be a clear coating composition or may be a coloredcoating composition. Method (d) is described below as a coating filmformation method in addition to the above methods.

Method (d): A one-coat method of forming a coating film, wherein asubstrate is coated with the coating composition of the presentinvention in a single layer, and the coating composition is cured.

The steps of forming a coating film in methods (a), (b), (c) and (d) aredescribed below in detail.

In the above method (a), the coating compositions for forming thecolored base coat encompass colored coating compositions and lustercoating compositions.

The colored base coating composition is an organic solvent-based oraqueous coating composition containing a base resin, a crosslinkingagent for the resin and a coloring pigment.

Examples of base resins include acrylic resins, vinyl resins, polyesterresins, alkyd resins, urethane resins, etc., and at least one base resinis used. Crosslinkable functional groups in the resins are, for example,hydroxyl group, epoxy group, carboxyl group, alkoxysilyl groups, etc.Examples of crosslinking agents include alkyl-etherified melamineresins, urea resins, guanamine resins, polyisocyanate compounds, blockedpolyisocyanate compounds, epoxy compounds, carboxyl-containingcompounds, etc., and at least one crosslinking agent is used. Theproportions of base resin and crosslinking agent are preferably 50% to90% by weight of base resin and 50% to 10% by weight of crosslinkingagent, based on the total amount of the two components.

To the colored base coating composition are added coloring pigmentsand/or effect pigments, etc., which are mentioned above as the pigmentsusable for the coating composition of the present invention.

In method (a), the substrate is coated with the colored base coatingcomposition to a film thickness of about 10 to about 50 μm (when cured)by a coating method such as airless spray coating, air spray coating,rotary atomization coating, etc. In these coating methods, anelectrostatic charge may be optionally applied. After coating, the basecoating composition is either cured by heating at about 100° C. to about180° C., and preferably at about 120° C. to about 160° C., for about 10to about 40 minutes, or is not cured with the substrate being left tostand at room temperature for several minutes or being preheated atabout 40° C. to about 100° C. for about 1 to about 20 minutes.

To form a top clear coat, the substrate is further coated with the clearcoating composition of the present invention to a film thickness ofabout 10 to about 70 μm (when cured) by a coating method as mentionedabove. The coated substrate is then heated and thereafter irradiatedwith light, or is then irradiated with light and thereafter heated. Thusa cured multilayer coating film can be formed. Preferably, heating isperformed at about 100° C. to about 180° C., especially at about 120° C.to about 160° C., for about 5 to about 30 minutes to cure the coatingfilm by crosslinking. Light irradiation is performed under theabove-described conditions of wavelength, light source, irradiationintensity and irradiation time.

Hereinafter, the method of applying a clear coating composition withoutheat-curing the base coating composition, and curing the two coatssimultaneously may be referred to as a two-coat one-bake system (2C1B).The method of heat-curing the base coating composition, then applying aclear coating composition and curing the clear coat may be referred toas a two-coat two-bake system (2C2B).

The above-described colored base coating composition for method (a) canbe used as the colored base coating composition for method (b). Anycoating composition for forming a clear coating film can be used as thefirst clear coating composition to form a clear coat. For example, acoating composition that contains little or no pigment used in thecolored base coating composition can be used. The coating composition ofthe present invention is used as the clear coating composition forforming a top clear coat. In addition, the clear coating composition ofthe invention may also be used as the first clear coating composition,so that the clear coat and the top clear coat are formed from the clearcoating composition of the invention.

In method (b), as in method (a), the colored base coating composition isapplied and either cured by heating, or not cured with the substratebeing left to stand at room temperature for several minutes or beingpreheated. The colored base coating film is then coated with the firstclear coating composition to a film thickness of about 10 to about 50 μm(when cured) by a coating method as mentioned above. The coatingcomposition is either cured by heating at about 100° C. to about 180°C., and preferably at about 120° C. to about 160° C., for about 10 toabout 40 minutes, or is not cured with the substrate being left to standat room temperature for several minutes or being preheated.

The substrate is then further coated with the coating composition of thepresent invention as a second clear coating composition to a coatingfilm thickness of about 10 to about 50 μm (when cured) by a coatingmethod as mentioned above. The coated substrate is then first heated andthereafter irradiated with light, or is then irradiated with light andthereafter heated. Thus a cured multilayer coating film can be formed.The conditions for heating and irradiation are the same as in method(a).

Hereinafter, the method of applying a first clear coating compositionwithout heat-curing the base coating composition, applying a secondclear coating composition without heat-curing the first clear coatingcomposition, and then curing the three coats simultaneously may bereferred to as a three-coat one-bake system (3C1B). The method ofapplying a first clear coating composition without heat-curing the basecoating composition, curing these coats simultaneously, and thenapplying and curing a second clear coating composition may be referredto as a three-coat two-bake system (3C2B). Moreover, the method ofheat-curing a base coating composition, applying and curing a firstclear coating composition, and then applying and curing a second clearcoating composition may be referred to as a three-coat three-bake system(3C3B).

The above-described colored base coating composition for method (a) canbe used as the first colored base coating composition for method (c).The second colored base coating composition is applied onto the coat ofthe first colored base coating composition; therefore, the secondcolored base coating composition is usually a clear colored compositionthat has weak hiding power such that the color tone of the first coloredcoating surface is visible through the second colored base coating film.

Hence, the second colored base coating composition is preferablyprepared by selecting a suitable type of pigment and adjusting theamount thereof in view of the first colored base coating compositionsuch that the hiding power of the second colored base composition isweaker than that of the first colored base composition. The coatingcomposition of the present invention is used as the clear coatingcomposition for the top clear coat.

In method (c), as in method (a), the first colored base coatingcomposition is applied and either cured by heating, or not cured withthe substrate being left to stand at room temperature for severalminutes or being preheated. The first colored base coating film is thencoated with the second colored base coating composition to a filmthickness of about 10 to about 50 μm (when cured) by a coating method asmentioned above. The coating composition is either cured by heating atabout 100° C. to about 180° C., and preferably at about 120° C. to about160° C., for about 10 to about 40 minutes, or is not cured with thesubstrate being left to stand at room temperature for several minutes orbeing preheated.

The substrate is then further coated with the coating composition of theinvention as a top clear coating composition to a film thickness ofabout 10 to about 50 μm (when cured) by a coating method as mentionedabove. The coated substrate is then heated and thereafter irradiatedwith light, or is then irradiated with light and thereafter heated. Thusa cured multilayer coating film can be formed. The conditions forheating and irradiation are the same as in method (a).

Hereinafter, the method of applying a second base coating compositionwithout heat-curing the first base coating composition, applying a clearcoating composition without heat-curing the second base coatingcomposition, and then curing the three coats simultaneously may bereferred to as a three-coat one-bake system (3C1B). The method ofheat-curing a first base coating composition, applying a second basecoating composition, applying a clear coating composition withoutheat-curing the second base coating composition, and then curing thesecoats simultaneously may be referred to as a three-coat two-bake system(3C2B). Moreover, the method of heat-curing a first base coatingcomposition, applying and curing a second base coating composition, andthen applying and curing a clear coating composition may be referred toas a three-coat three-bake system (3C3B).

In method (d), the substrate is coated with the coating composition ofthe present invention to a film thickness of about 10 to about 50 μm(when cured) by the same coating method as in method (a). The coatedsubstrate is either heated and thereafter irradiated with light, or isirradiated with light and thereafter heated. Thus a cured single-layercoating film can be formed. The conditions for heating and irradiationare the same as in method (a). This method may be hereinafter referredto as a one-coat one-bake system (1C1B).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the Vickers hardness of the coating filmobtained in Example 5, measured by a micro hardness tester(“Fischerscope H-100”, product of Fischer Instruments K.K.). In FIG. 1,bracketed range 1 indicates the indentation recovery.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is described below in more detail with referenceto Examples and Comparative Examples. However, the present invention isnot limited by these examples. In the examples, parts and percentagesare expressed on a weight basis.

Production of Compound (A)

PRODUCTION EXAMPLE 1 Production of Resin ContainingRadical-polymerizable Unsaturated Groups

Eight hundred and eighty-eight parts of isophorone diisocyanate, 464parts of 2-hydroxyethyl acrylate and 0.7 parts of hydroquinonemonomethyl ether were placed into a reaction vessel equipped with athermometer, a thermostat, a stirrer, a reflux condenser and an airblower. While blowing air into the reaction vessel, the mixture washeated to 80° C. and maintained at the same temperature for 5 hours.After confirming substantially all the 2-hydroxyethyl acrylate hadreacted, an adduct of isophorone diisocyanate and 2-hydroxyethylacrylate was obtained. Subsequently, 136 parts of pentaerythritol, 372parts of butyl acetate and 0.2 parts of dibutyltin dilaurate were addedto the adduct, and the mixture was further maintained at 80° C. Afterconfirming substantially all the isophorone diisocyanate had reacted,the mixture was cooled, giving a solution of resin (resin No. 1)containing radical-polymerizable unsaturated groups with a resin solidscontent of 80%. The resin had a number average molecular weight of about1,500 and about four radical-polymerizable unsaturated groups permolecule.

PRODUCTION EXAMPLE 2 Production of Resin ContainingRadical-polymerizable Unsaturated Groups and Hydroxyl Groups

Four hundred and eighty parts of butyl acetate was placed into areaction vessel equipped with a thermometer, a thermostat, a stirrer, areflux condenser and a dropping funnel, and was heated to 130° C. whilefeeding nitrogen gas into the reaction vessel. While maintaining thesame temperature, a mixed solution of the following monomers andpolymerization initiator was added dropwise over 3 hours via thedropping funnel.

styrene 200 parts methyl methacrylate 250 parts cyclohexyl methacrylate200 parts 2-hydroxyethyl methacrylate 350 parts2,2′-azobis(2-methylbutyronitrile)  50 parts

After completion of the dropwise addition, the mixture was aged at 130°C. for 1 hour, giving a hydroxyl-containing copolymer solution with aresin solids content of 70%. The obtained resin was analyzed by gelpermeation chromatography with the result that it had a number averagemolecular weight of about 8,000. The resin had a hydroxyl value of 138mg KOH/g.

To this resin were added 338 parts of the adduct of isophoronediisocyanate and 2-hydroxyethyl acrylate obtained in Production Example1, 0.4 parts of hydroquinone monomethyl ether, 145 parts of butylacetate and 0.2 parts of dibutyltin dilaurate. While blowing air intothe reaction vessel, the mixture was heated to 80° C. and maintained atthe same temperature for 5 hours. After confirming substantially all theisocyanate groups had reacted, the mixture was cooled, giving a solutionof resin (resin No. 2) containing radical-polymerizable unsaturatedgroups and hydroxyl groups with a resin solids content of 70%. The resinhad a number average molecular weight of about 10,500, aradical-polymerizable unsaturated group content of 0.72 mol/kg and ahydroxyl value of 68 mg KOH/g.

Production of Hydroxyl-containing Polyester Resin (B)

PRODUCTION EXAMPLE 3

Monomer mixture 1 (Note 1) was placed into a four-necked flask equippedwith a heater, a stirrer, a thermometer and a reflux condenser, and washeated to 160° C. The contents of the flask were heated from 160° C. to230° C. over 3 hours, and maintained at 230° C. for 1 hour. The producedcondensation water was distilled off using a rectification column, and 5parts of xylene was added to the contents of the flask. Xylene andcondensation water was refluxed, removing the water by a waterseparator.

Two hours after the addition of xylene, the measurement of acid valuewas started. When the acid value reached 2 mg KOH/g, the mixture wascooled to 140° C. Phthalic anhydride was then added to the mixture,which was maintained at 140° C. for 2 hours to perform an additionreaction. The reaction mixture was thereafter cooled to terminate thereaction. Mixed solvent A (Note 2) was further added, giving polyesterresin No. 1 with a solids content of 70%. Polyester resin No. 1 had aweight average molecular weight of 28,000 and a hydroxyl value of 157 mgKOH/g.

(Note 1) Monomer mixture 1: a mixture of 1,4-cyclohexanedimethanol (74parts), trimethylolpropane (65 parts), 1,4-cyclohexanecarboxylic acid(66 parts), adipic acid (74 parts) and dibutyltin dilaurate (0.5 parts)

(Note 2) Mixed solvent A: a mixture of xylene/“Swasol 1000”(product ofCosmo Oil Co., Ltd., high-boiling petroleum solvent) =50/50 (weightratio)

PRODUCTION EXAMPLES 4 AND 5

Except that the monomer components shown in Table 1 were used, theprocedure of Production Example 3 was repeated to obtain polyester resinNo. 2 and polyester resin No. 3, each with a solids content of 70%.Polyester resin No. 2 had a weight average molecular weight of 32,000and a hydroxyl value of 150 mg KOH/g. Polyester resin No. 3 had a weightaverage molecular weight of 30,000 and a hydroxyl value of 172 mg KOH/g.

Table 1 shows the monomer components of polyester resin No. 1 to No. 3,and the proportion of alicyclic polybasic acid and/or alicyclicpolyhydric alcohol to the monomers in total.

TABLE 1 Polyester resin No. 1 2 3 1,4-cyclohexanedimethanol 74 74Trimethylolpropane 65 65 65 Neopentyl glycol 531,4-cyclohexanedicarboxylic acid 66 153 Phthalic anhydride 56 Adipicacid 74 74 Dibutyltin dilaurate 0.5 0.5 0.5 Proportion of alicyclicpolybasic 50.2 77.7 0 acid and alicyclic polyhydric alcohol to themonomers in total (%) Proportion of alicyclic polyhydric 53.2 53.2 0alcohol to polyhydric alcohol (%) Proportion of alicyclic polybasic acid47.1 100 0 to polybasic acid (%)Production of Hydroxyl-containing Acrylic Resin

PRODUCTION EXAMPLE 6

Four hundred and eighty parts of butyl acetate was placed into areaction vessel equipped with a thermometer, a thermostat, a stirrer, areflux condenser and a dropping funnel, and was heated to 130° C. whilefeeding nitrogen gas into the reaction vessel. While 10 maintaining thesame temperature, a mixed solution of the following monomers andpolymerization initiator was added dropwise over 3 hours via thedropping funnel.

styrene 200 parts methyl methacrylate 290 parts cyclohexyl methacrylate250 parts 2-hydroxyethyl methacrylate 260 parts2,2′-azobis(2-methylbutyronitrile)  50 parts

After completion of the dropwise addition, the mixture was aged at 130°C. for 1 hour, giving a solution of hydroxyl-containing acrylic resinwith a resin solids content of 70%. The obtained resin was analyzed bygel permeation chromatography with the result that it had a numberaverage molecular weight of about 8,000. The resin had a hydroxyl valueof 107 mg KOH/g.

Production of Coating Composition

EXAMPLE 1

Thirty-five parts of resin No. 1 obtained in Production Example 1, 100parts of polyester resin No. 1 obtained in Production Example 3, and 28parts of “Sumidur N-3300”(Note 3) were mixed with stirring. After 3parts of “IRGACURE 184”(Note 6) was added and dissolved in the mixture,1 part of “TINUVIN 400”(Note 8) and 1 part of “SANOL LS-292”(Note 9)were further added and dissolved in the mixture. The resulting mixturewas diluted with xylene to adjust its viscosity to 25 seconds at 20° C.(viscometer: Ford cup No. 4), giving clear coating composition No. 1 ofthe present invention with a solids content of 50%.

EXAMPLES 2 TO 4 AND COMPARATIVE EXAMPLES 1 TO 4

Except that the components shown in Table 2 were used, the procedure ofExample 1 was repeated to obtain clear coating compositions No. 2 to No.4 of the present invention in Examples 2 to 4, and comparative clearcoating compositions No. 5 to No. 8 in Comparative Examples 1 to 4.

Table 2 shows the proportions of the components of coating compositionsNo. 1 to No. 8 on a solids basis and the solids content of thecompositions.

TABLE 2 Coating composition No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7No. 8 Resin No. 1 in 35 35 35 Production Ex. 1 Resin No. 2 in 35Production Ex. 2 Pentaerythritol 35 tetraacrylate Dipenta- 32 100erythritol hexaacrylate Polyester resin 100 100 100 No. 1 Polyesterresin 100 No. 2 Polyester resin 100 No. 3 Acrylic resin in 100 100Production Ex. 6 Sumidur 28 28 30 20 28 36 N-3300 (Note 3) Cymel 325 8(Note 4) Cymel 235 36 (Note 5) IRGACURE 3 3 2 2 3 2.5 3 184 (Note 6)IRGACURE 0.5 0.5 0.5 819 (Note 7) TINUVIN 400 1 1 1.5 0.7 1 1 1.3 1(Note 8) SANOL 1 1 1.5 1.5 1 1.5 1.3 1 LS-292 (Note 9) solids 50 45 5251 52 52 50 85 content (%) In Table 2, (Note 3) to (Note 9) indicate thefollowing: (Note 3) Sumidar N-3300: trade name, product of Sumika BayerUrethane Co. Ltd., isocyanurate-modified hexamethylene diisocyanate,solids: 100% (Note 4) Cymel 325: trade name, product of Mitsui CytecLtd., methylated/imino melamine resin (Note 5) Cymel 235: trade name,product of Mitsui Cytec Ltd., methylated/butylated melamine resin (Note6) IRGACURE 184: trade name, product of Ciba Specialty Chemicals K. K.,photopolymerization initiator (Note 7) IRGACURE 819: trade name, productof Ciba Specialty Chemicals K. K., photopolymerization initiator (Note8) TINUVIN 400: trade name, Ciba Speciality Chemicals K. K., triazineultraviolet absorber (Note 9) SANOL LS-292: trade name, Sankyo Co.,Ltd., light stabilizer, bis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate

PRODUCTION EXAMPLE 7

Production of Test Sheet

A zinc phosphate-treated galvanized steel sheet (length: 25 cm, width:25 cm, thickness: 0.8 mm) was coated with a cationic electrodepositioncoating composition (trade name: “Elecron GT-10LF”, product of KansaiPaint Co., Ltd.) by electrodeposition to a film thickness of about 20 μm(when cured). The coated sheet was heated at 170° C. for 20 minutes tocure the coating film. The sheet was further coated with a coatingcomposition for an automobile intermediate coat (trade name: “AmilacTP-65-2”, product of Kansai Paint Co., Ltd.) by air spraying to a filmthickness of about 35 μm (when cured). The coated sheet was heated at140° C. for 30 minutes to cure the coating film, thus giving a testsheet.

Coating Film Formation Method

EXAMPLE 5

The test sheet obtained in Production Example 7 was coated with awaterborne colored coating composition for a base coat (trade name:“WBC-710T (black)”, product of Kansai Paint Co., Ltd., acrylicresin/melamine resin thermosetting coating composition) to a filmthickness of about 15 μm (when cured) and was dried at 80° C. for 10minutes to vaporize the water in the coating film. The resulting uncuredcoating surface was coated with a clear coating composition (trade name:“Magicron TC-69”, product of Kansai Paint Co., Ltd., acrylicresin/melamine resin thermosetting coating composition) to a filmthickness of about 35 μm (when cured). The coated sheet was heated at140° C. for 30 minutes to simultaneously cure the base coating film andthe clear coating film.

The cured clear coating film was coated with coating composition No. 1obtained in Example 1 by air spraying to a film thickness of about 40 μm(when cured). The resulting coating film was dried at 90° C. for 3minutes in a preheating step to evaporate the solvent. Subsequently,using a 120 W/cm metal halide lamp as a light source, the coating filmwas irradiated for photocuring with ultraviolet light (wavelength: about365 nm) for about 10 seconds at an intensity of 1,000 mJ/cm², andthereafter heated at 140° C. for 5 minutes to cure the coating film.

Thus a multilayer coating film was formed by the three-coat two-bakesystem.

EXAMPLE 6

In a similar manner as in Example 5 except that the coating film afterphotocuring was heated at 140° C. for 30 minutes, a multilayer coatingfilm was formed by the three-coat two-bake system.

EXAMPLE 7

In a similar manner as in Example 5 except that coating composition No.2 obtained in Example 2 was used in place of coating composition No. 1,a multilayer coating film was formed by the three-coat two-bake system.

EXAMPLE 8

In a similar manner as in Example 5 except that coating composition No.3 obtained in Example 3 was used in place of coating composition No. 1,a multilayer coating film was formed by the three-coat two-bake system.

EXAMPLE 9

In a similar manner as in Example 5 except that coating composition No.4 obtained in Example 4 was used in place of coating composition No. 1,and that the coating film after photocuring was heated at 140° C. for 10minutes, a multilayer coating film was formed by the three-coat two-bakesystem.

EXAMPLE 10

In a similar manner as in Example 6 except that coating composition No.4 obtained in Example 4 was used in place of coating composition No. 1,a multilayer coating film was formed by the three-coat two-bake system.

EXAMPLE 11

The test sheet obtained in Production Example 7 was coated with awaterborne colored coating composition for a base coat (trade name:“WBC-710T (black)”, product of Kansai Paint Co., Ltd., acrylicresin/melamine resin thermosetting coating composition) to a filmthickness of about 15 μm (when cured) and was dried at 80° C. for 10minutes to vaporize the water in the coating film. The resulting uncuredcoating surface was coated with a clear coating composition (trade name:“Magicron TC-69”, product of Kansai Paint Co., Ltd., acrylicresin/melamine resin thermosetting coating composition) to a filmthickness of about 35 μm (when cured).

The uncured clear coating film was coated with coating composition No. 1obtained in Example 1 by air spraying to a film thickness of about 40 μm(when cured). The resulting coating film was heated at 140° C. for 30minutes. Subsequently, using a 120 W/cm metal halide lamp as a lightsource, the coating film was irradiated for photocuring with ultravioletlight (wavelength: about 365 nm) for about 10 seconds at an intensity of1,000 mJ/cm² to simultaneously cure the three-layer coating film.

Thus a multilayer coating film was formed by the three-coat one-bakesystem.

EXAMPLE 12

The test sheet obtained in Production Example 7 was coated with awaterborne colored coating composition for a base coat (trade name:“WBC-710T (black)”, product of Kansai Paint Co., Ltd., acrylicresin/melamine resin thermosetting coating composition) to a filmthickness of about 15 μm (when cured) and was dried at 80° C. for 10minutes to vaporize the water in the coating film.

The uncured coating surface was coated with coating composition No. 1obtained in Example 1 by air spraying to a film thickness of about 40 μm(when cured). The resulting coating film was dried at 90° C. for 3minutes in a preheating step to evaporate the solvent. Subsequently,using a 120 W/cm metal halide lamp as a light source, the coating filmwas irradiated for photocuring with ultraviolet light (wavelength: about365 nm) for about 10 seconds at an intensity of 1,000 mJ/cm², andthereafter heated at 140° C. for 30 minutes to simultaneously cure thetwo-layer coating film.

Thus a multilayer coating film was formed by the two-coat one-bakesystem.

Table 3 shows the top clear coating compositions, coating film formationsteps and film-curing conditions used in Examples 5 to 12.

TABLE 3 Example 5 6 7 8 9 10 11 12 Coating composition No. 1 No. 1 No. 2No. 3 No. 4 No. 4 No. 1 No. 1 Coating film formation step 3C2B 3C2B 3C2B3C2B 3C2B 3C2B 3C1B 2C1B Irradiation Intensity(mJ/cm²) 1,000 1,000 1,0001,000 1,000 1,000 1,000 Time(sec) 10 10 10 10 10 10 10 HeatingTemperature(° C.) 140 140 140 140 140 140 140 140 Time(min) 5 30 5 5 1030 30 30 Irradiation Intensity(mJ/cm²) 1,000 Time(sec) 10

COMPARATIVE EXAMPLE 5

In a similar manner as in Example 5 except that coating composition No.5 obtained in Comparative Example 1 was used in place of coatingcomposition No. 1, a multilayer coating film was formed by thethree-coat two-bake system.

COMPARATIVE EXAMPLE 6

In a similar manner as in Example 5 except that coating composition No.6 obtained in Comparative Example 2 was used in place of coatingcomposition No. 1, a multilayer coating film was formed by thethree-coat two-bake system.

COMPARATIVE EXAMPLE 7

The test sheet obtained in Production Example 7 was coated with awaterborne colored coating composition for a base coat (trade name:“WBC-710T (black)”, product of Kansai Paint Co., Ltd., acrylicresin/melamine resin thermosetting coating composition) to a filmthickness of about 15 μm (when cured) and was dried at 80° C. for 10minutes to vaporize the water in the coating film. The resulting uncuredcoating surface was coated with a clear coating composition (trade name:“Magicron TC-69”, product of Kansai Paint Co., Ltd., acrylicresin/melamine resin thermosetting coating composition) to a filmthickness of about 35 μm (when cured). The coated sheet was heated at140° C. for 30 minutes to simultaneously cure the base coating film andthe clear coating film.

The cured clear coating film was coated with coating composition No. 7obtained in Comparative Example 3 by air spraying to a film thickness ofabout 40 μm (when cured). The resulting coating film was cured only byheating at 140° C. for 5 minutes without ultraviolet irradiation. Thus amultilayer coating film was formed by the three-coat two-bake system.

COMPARATIVE EXAMPLE 8

The test sheet obtained in Production Example 7 was coated with awaterborne colored coating composition for a base coat (trade name:“WBC-710T (black)”, product of Kansai Paint Co., Ltd., acrylicresin/melamine resin thermosetting coating composition) to a filmthickness of about 15 μm (when cured) and was dried at 80° C. for 10minutes to vaporize the water in the coating film. The resulting uncuredcoating surface was coated with a clear coating composition (trade name:“Magicron TC-69”, product of Kansai Paint Co., Ltd., acrylicresin/melamine resin thermosetting coating composition) to a filmthickness of about 35 μm (when cured). The coated sheet was heated at140° C. for 30 minutes to simultaneously cure the base coating film andthe clear coating film.

The cured clear coating film was coated with coating composition No. 7obtained in Comparative Example 3 by air spraying to a film thickness ofabout 40 μm (when cured). The resulting coating film was cured only byheating at 140° C. for 30 minutes without ultraviolet irradiation. Thusa multilayer coating film was formed by the three-coat two-bake system.

COMPARATIVE EXAMPLE 9

The test sheet obtained in Production Example 7 was coated with awaterborne colored coating composition for a base coat (trade name:“WBC-710T (black)”, product of Kansai Paint Co., Ltd., acrylicresin/melamine resin thermosetting coating composition) to a filmthickness of about 15 μm (when cured) and was dried at 80° C. for 10minutes to vaporize the water in the coating film. The resulting uncuredcoating surface was coated with a clear coating composition (trade name:“Magicron TC-69”, product of Kansai Paint Co., Ltd., acrylicresin/melamine resin thermosetting coating composition) to a filmthickness of about 35 μm (when cured). The coated sheet was heated at140° C. for 30 minutes to simultaneously cure the base coating film andthe clear coating film.

The cured clear coating film was coated with coating composition No. 8obtained in Comparative Example 4 by air spraying to a film thickness ofabout 40 μm (when cured). The resulting coating film was dried at 90° C.for 3 minutes in a preheating step to evaporate the solvent.Subsequently, using a 120 W/cm metal halide lamp as a light source, thecoating film was irradiated with ultraviolet light (wavelength: about365 nm) for about 10 seconds at an intensity of 1,000 mJ/cm² tophotocure the coating film. Thus a multilayer coating film was formed bythe three-coat two-bake system.

COMPARATIVE EXAMPLE 10

The test sheet obtained in Production Example 7 was coated with awaterborne colored coating composition for a base coat (trade name:“WBC-710T (black)”, product of Kansai Paint Co., Ltd., acrylicresin/melamine resin thermosetting coating composition) to a filmthickness of about 15 μm (when cured) and was dried at 80° C. for 10minutes to vaporize the water in the coating film.

The resulting uncured coating surface was coated with coatingcomposition No. 5 obtained in Comparative Example 1 by air spraying to afilm thickness of about 40 μm (when cured). The resulting coating filmwas dried at 90° C. for 3 minutes in a preheating step to evaporate thesolvent. Subsequently, using a 120 W/cm metal halide lamp as a lightsource, the coating film was irradiated for photocuring with ultravioletlight (wavelength: about 365 nm) for about 10 seconds at an intensity of1,000 mJ/cm², and thereafter heated at 140° C. for 30 minutes tosubstantially fully cure the coating film. Thus a multilayer coatingfilm was formed by the two-coat one-bake system.

Table 4 shows the top clear coating compositions, 5 coating filmformation steps and film-curing conditions used in Comparative Examples5 to 10.

TABLE 4 Comparative Example 5 6 7 8 9 10 Coating composition No. 5 No. 6No. 7 No. 7 No. 8 No. 5 Coating film formation 3C2B 3C2B 3C2B 3C2B 3C2B2C1B step Irradiation Intensity 1,000 1,000 1,000 1,000 (mJ/cm²)Time(sec) 10 10 10 10 Heating Temperature 140 140 140 140 140 (° C.)Time(min) 5 5 5 30 30

The multilayer coating films obtained by the coating film formationmethods of Examples 5 to 12 and Comparative Examples 5 to 10 were eachtested for their film properties as follows:

Degree of curing of coating film: The coating surface was wiped for 50reciprocatory strokes with a gauze soaked with xylene and was observedto evaluate the degree of curing of the coating film according to thefollowing criteria:

-   -   A: No changes occur on the coating surface. The coating film is        sufficiently cured.    -   B: The coating surface is marred. The coating film is not        sufficiently cured.    -   C: The coating surface is dissolved by the xylene. The curing of        the coating film is far from sufficient.

Mar resistance: Polishing sand (trade name: “Daruma cleanser”, productof Yamasan-shouten Ltd.) mixed with an equal weight of water was placedon the coated sheet. While being pressed with the flannel-covered testerterminal of a friction tester (product of Suga Test Instruments Co.,Ltd., model: FR-2S), the coated sheet was rubbed back and forth for 25reciprocating strokes under a load of 0.5 kg. The coating surface wasobserved to evaluate the mar resistance of the coating film according tothe following criteria:

-   -   A: No changes are observed in the gloss of the coating surface.        The mar resistance of the coating film is excellent.    -   B: The gloss of the coating surface is impaired. The mar        resistance of the coating film is poor.    -   C: The gloss of the coating surface is severely impaired. The        mar resistance of the coating film is very poor.

Knoop hardness: After the coated sheet was left in a thermostatic roomat 20° C. for 4 hours, measurement was conducted using a Tukon hardnesstester (TUKON microhardness tester, product of American Chain & CableCompany).

Vickers hardness: Using a micro hardness tester (trade name:“Fischerscope H-100”, product of Fischer Instruments K.K.), a loadincreasing from 0.4 mN to 10 mN over 50 seconds was applied to thecoating film surface with a diamond pyramid indenter. In the subsequent50 seconds, the load was decreased from 10 mN to 0.4 mN. The load wasthen maintained at 0.4 mN for 100 seconds to determine the indentationrecovery value (μm). The larger the recovery value (μm) is, the greaterelasticity the coating film has. FIG. 1 is a graph showing the Vickershardness of the multilayer coating film obtained in Example 5. In FIG.1, bracketed range 1 indicates the indentation recovery measured duringthe 100-second period.

Adhesion: After the coated sheet was immersed in warm water at 40° C.for 240 hours, the cured coating film was given crosscuts with a cutterreaching down to the substrate so as to form a grid of 100 squares (2 mmx 2 mm). Adhesive tape was applied to the surface of the grid portionand forcefully pulled upward. Peeling of the top clear coating film wasinspected, counting the number of remaining crosscut squares. Adhesionwas evaluated in terms of the number of remaining crosscut squares per100 crosscut squares.

Table 5 shows the test results.

TABLE 5 Example Comparative Example 5 6 7 8 9 10 11 12 5 6 7 8 9 10Degree of curing of A A A A A A A A A A C A A A coating film Marresistance A A A A A A A A A A C B A A Knoop hardness 5 7 5.5 5.5 7 9 67 2 7 1 8 20 2 Vickers hardness 0.7 0.6 0.6 0.5 0.5 0.5 0.7 0.6 0.3 0.20.1 0.3 0 0.3 Adhesion 100 100 100 100 100 100 100 100 100 80 0 100 0100

The present invention achieves the following remarkable effects.

Because of its thermosetting and photocuring properties, the coatingcomposition of the present invention can shorten the heating time forcuring a coating film, thereby reducing the time for forming a curedcoating film and the size of coating facilities, etc. Thus the coatingcomposition of the invention can contribute to saving space and energy,and can reduce CO₂ and other factors of environmental contamination.

The coating film obtained from the coating composition of the inventionhas excellent properties in terms of mar resistance, hardness, adhesion,etc. It is considered that these excellent film properties are achievedbecause the coating composition comprises a specific hydroxyl-containingpolyester resin in combination with a specific radical-polymerizablecompound, a crosslinking agent and a photopolymerization initiator; theresulting cured coating film has improved elasticity as well assufficient hardness, thus providing excellent film properties in termsof mar resistance, etc.

In the coating film formation method according to the present invention,the coating composition of the invention is used for forming a top clearcoat, so that a multilayer coating film is suitably formed on asubstrate such as an automobile body by a two-coat one-bake system,two-coat two-bake system, three-coat one-bake system, three-coattwo-bake system, etc. The obtained multilayer coating film has excellentproperties in mar resistance, hardness, adhesion, etc. In the case of anautomobile body, therefore, the present invention provides greatlyimproved resistance to marring by car washes, scratching aroundkeyholes, etc.

1. A thermosetting and photocurable coating composition for automobilebodies comprising: (A) at least one compound selected from the groupconsisting of urethane resins containing radical-polymerizableunsaturated groups, and acrylic resins containing radical-polymerizableunsaturated groups and hydroxyl groups; (B) a hydroxyl-containingpolyester resin produced by esterifying a polybasic acid (a) with apolyhydric alcohol (b), wherein an alicyclic polybasic acid (a1) and analicyclic polyhydric alcohol (b1) are included in a total ratio of 50%to 100% based on the total weight of polybasic acid (a) and polyhydricalcohol (b), the polyhydric alcohol (b) includes 50% or more of thealicyclic polyhydric alcohol (b1) based on the weight of the polyhydricalcohol (b) the alicyclic polybasic acid (a1) is at least one compoundselected from the group consisting of cyclohexane-1,3-dicarboxylic acid,cyclohexane-1,4-dicarboxylic acid, hexahydrophthalic acid,hexahydrotrimellitic acid, tetrahydrophthalic acid, methylhexahydrophthalic acid; and anhydrides of these acids; and the alicyclicpolyhydric alcohol (b1) is at least one compound selected from the groupconsisting of cyclohexane-1,4-dimethylol, hydrogenated bisphenol-A,spiroglycol, and dihydroxymethyltricyclodecane; (C) a crosslinkingagent; and (D) a photopolymerization initiator.
 2. A coating compositionaccording to claim 1, wherein the polybasic acid (a) includes 50% ormore of alicyclic polybasic acid (a1) based on the weight of polybasicacid (a).
 3. A coating composition according to claim 1, wherein thepolyester resin (B) has a hydroxyl value of 20 to 800 mg KOH/g.
 4. Acoating composition according to claim 1, wherein the crosslinking agent(C) is a polyisocyanate compound.
 5. A coating composition according toclaim 1, wherein the crosslinking agent (C) is a combination of apolyisocyanate compound and a melamine resin.
 6. A coating compositionaccording to claim 1, comprising, per 100 parts by weight of polyesterresin (B), about 5 to about 200 parts by weight of compound (A), about 5to about 200 parts by weight of crosslinking agent (C) and about 0.1 toabout 20 parts by weight of photopolymerization initiator (D).
 7. Acoating composition according to claim 1, further comprising a lightstabilizer (E).
 8. A coating composition according to claim 1, furthercomprising an ultraviolet absorber (F).
 9. A coating compositionaccording to claim 1, wherein the coating composition is an organicsolvent-based composition and has a solids content of about 20% to about90% by weight.
 10. A method of forming a coating film comprising formingone or more colored base coats and one or more clear coats on asubstrate to be coated to form a multilayer coating film, the top clearcoat being formed from the coating composition according to claim 1,wherein the top clear coat is cured either by irradiating with lightafter heating or by heating after irradiating with light.
 11. A methodof forming a coating film according to claim 10, wherein a colored basecoat and a top clear coat are formed on the substrate to form amultilayer coating film according to a two-coat system.
 12. A method offorming a coating film according to claim 10, wherein a colored basecoat, a clear coat and a top clear coat are formed on the substrate toform a multilayer coating film according to a three-coat system.
 13. Amethod of forming a coating film according to claim 10, wherein a firstcolored base coat, a second colored base coat and a top clear coat areformed on the substrate to form a multilayer coating film according to athree-coat system.
 14. A method of forming a coating film according toclaim 10, wherein the top clear coat is heated at about 100° C. to about180° C. for about 5 to about 30 minutes.
 15. A method of forming acoating film according to claim 10, wherein the top clear coat isirradiated with ultraviolet light having a wavelength of about 200 toabout 450 nm at an intensity of about 100 to about 5,000 mJ/cm².
 16. Amethod of forming a coating film according to claim 10, wherein thesubstrate to be coated is a vehicle body.
 17. A method of forming acoating film according to claim 16, wherein the substrate to be coatedis an automobile body.